One type of common prior art brake design for vehicles is a two piece rotor and hub in which a rotor that carries the braking surface is detachably connected to a wheel hub. Another common type of brake design is an integrated one-piece rotor and hub assembly.
Integrated one-piece rotor and hub assemblies have the advantage that no fasteners are required between the rotor and the hub. As a result, the integrated assemblies do not face problems associated with fasteners such as wear and fatigue near fastener openings and potential misalignment due to imperfect machining. A significant drawback, however, is that the assembly is constrained at the hub, which causes thermal distortion of the rotor. During braking, the rotor in such an assembly is subjected to high frictional forces that generate heat in the rotor causing thermal expansion/distortion, temperature variation across the face of the rotor, and heat transfer to the adjacent components including the hub and the bearings. Thermal expansion of the rotor is very limited because of the integral connection between the rotor and the hub, which results in thermal coning of the rotor surface and a large thermal gradient, which will induce high thermal stress leading to thermal cracking. The high thermal gradient generated during braking and the effects of the thermal expansion and distortion can cause vibration and thermal judder across the brake surfaces, resulting in rough or irregular braking pulsations, reduce the life and performance of the rotor and increase maintenance costs. Such thermal distortion can damage the rotor and when the rotor is damaged or worn, the entire integrated assembly must be replaced. This is expensive and time consuming.
One way the thermal stresses have been addressed is to provide a “floating” rotor in which the fastener connection between the rotor and the hub is provided with a small clearance or float that allows thermal expansion. Advantageously, in these designs the rotor is mounted directly to the hub such that braking force is applied in-plane to the hub thus minimizing torsion or twist between the rotor and hub attachment, which can result in cracking and breaking of the rotor or hub.
Two-piece rotor/hub assemblies also allow greater flexibility with respect to use with different hubs, as the same rotor disc can be used with different hubs. This reduces the cost since generic rotor discs can be used and only the hub portion requires specialized casting, tooling and machining steps. Thus, floating rotor/hub assemblies reduce the necessity for complete replacement of a worn, cracked or distorted rotor, since the rotor disc can be detached from the hub for less expensive and easier replacement than with the integrated design. Other advantages of integrating the rotor mount with the hub is reduced weight as compared to rotor/hat designs, wherein the hat portion is usually made of iron for OEM vehicles, and the ease of assembly or replacing parts in the field.
However, stresses induced by conventional fastener assemblies in these designs are also a problem, even in floating rotor brakes. In most conventional designs a rotor attachment flange is held against the hub with a series of bolts or studs capped with nuts at a central portion of the rotor. The hub portion is placed on one side of the attachment flange and a fastener connects the hub portion to the side of the attachment flange. During braking, a frictional force is applied to the rotor surface, which creates torque that is transferred to the attachment flange, to the fastener, through the hub. Because the hub portion is attached to one side of the attachment flange, which is in a plane axially displaced from the friction braking surface, a moment arm is created at this connection joint. When the torque is transferred through a moment arm, bending stresses are formed in the connection. This creates twisting in the areas adjacent the fastener, which can create fatigue leading to cracking and breaking. These bolts or studs absorb and transfer a major amount of the braking force to the hub and are thus subject to intense thermal and bending stresses during braking.
Torque transfer also tends to be non-uniform through the perforated flange, especially in a floating design, as the machining tolerance at each aperture causes certain connections to receive more torque than other connections. This creates high stresses at individual apertures and can cause the attachment flange to crack or to have portions break off.
The two-piece hub/rotor assemblies discussed above also have drawbacks associated with the hub portion, which typically has slots that match with the perforations in the rotor attachment flange. Some floating type two piece hub/rotor assemblies use a spacer, sometimes called a bobbin, to provide the clearance that accommodates thermal expansion. The bobbin fits in the slots of the hub piece or in slots of the rotor flange, and when torque is applied to the hub through the rotor, the bobbin twists in the slot. This twisting causes the edges of the bobbin, which are typically square to match the slot, to gouge the sides of the slots, thus damaging the slotted piece. This is especially true when the hub piece is manufactured from a material having a lower hardness, such as aluminum, which is popular in high performance and racing applications, or when the rotor is formed of cast iron.
As such, these parts represent another weak link in the system, and can break-off during maintenance procedures. Additionally, holes drilled in the rotors for receiving these connectors can weaken the overall design and likewise crack due to the thermal and torsional forces created during braking.
However, despite recent advances, there remains an unmet need in the art to optimize and simplify attachment of floating disc brake rotors to wheel hubs.
There is a need, therefore, to provide a hub and rotor assembly that eliminates bending stresses and promotes uniform torque transfer, so as to minimize bending and fatigue stresses to increase the life and reliability of the brake device.